<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    div{
        width: 100px;
        height: 100px;
        background: salmon;
    }
    </style>
</head>
<body>
    <div id="tes" class="test"></div>
    <script>
        var test = document.getElementById("test") || document.getElementsByClassName("test")[0];
        console.log(test);
        test.onclick =  function(){
            this.style.backgroundColor ="skyblue"
        }
        var a = 10 ;
        var b =20 ;

    </script>
</body>
</html>